ASIA-PACIFIC TELECOMMUNITY
The First meeting of SATRC Working Group on Spectrum
07-08 July 2010, Tehran, I.R of IRAN
Document No.: SATRC-WG-SPEC-1/07
Date: 07 July 2010
Communication Regulatory Authority, I.R of IRAN
Cognative Radio and its Impacts on Spectrum Management
Mrs. Mina Dashti, Dr. Azim Fard
__________________
1. Introduction
Cognitive radio is widely expected to be the next Big Bang in wireless communications.
Spectrum regulatory Committees in many countries have been taking steps to open the door to
dynamic spectrum access using this technology and also laying down the rules for its
implementation. International organizations have also been striving for standardizing and
harmonization this technology for various applications. This document overviews the state of
art in the regulatory and standardization activities on cognitive radio all over the world, which
are deemed to have fundamental influence on the future of wireless communications.
Cognitive radio concepts can be applied to a variety of wireless communications scenarios, a
few of which are described in this document. Additionally, the major functions of cognitive
radio and components of cognitive radio and implementation issues are reviewed. We also
discuss the Regulatory Issues and Key Concepts. Finally, based on conducted survey through
the technical and regulatory investigation, a consistent conclusion provided that my help
administrations for drawing of their own policy guideline.
2. Background
Most of today’s radio systems are not aware of their radio spectrum environment and operate
in a specific frequency band using a specific spectrum access system. Investigations of
spectrum utilization indicate that not all the spectrum is used in space (geographic location) or
time. A radio, therefore, that can sense and understand its local radio spectrum environment,
to identify temporarily vacant spectrum and to use it, has the potential to provide wider
bandwidth, increase spectrum efficiency and minimize the need for centralized spectrum
management. This could be achieved by a radio that can make autonomous decisions about
how it accesses spectrum intelligently. Cognitive radios have the potential to do this.
The terms software-defined radio and cognitive radio were promoted by Mitola in 1991 and
1998, respectively. Software-defined radio is generally a multiband radio that supports
multiple air interfaces and protocols and is reconfigurable through software run on DSP or
Name:Mina Dashti
Tel:+98 21 8854481
Contact:
Organisation:CRA
Fax:+98 21 88468999
Country:I.R of IRAN
E-mail: dashti@cra.ir
SATRC-WG-SPEC-1/07
general-purpose microprocessors. Cognitive radio, built on a software radio platform, is a
context-aware intelligent radio potentially capable of autonomous reconfiguration by learning
from and adapting to the communication environment
It is important to note that the implementation of CRs technology will provide additional
capabilities to radiocommunication systems, such as dynamic spectrum access. Systems
which use some cognitive features have already been deployed and some administrations are
authorizing these systems. These administrations have national equipment approval processes
to protect existing services from harmful interference. However it should be noted that
services employing SDR or CRS technology will have to respect the sharing criteria for each
radiocommunication service given in the relevant ITU-R Recommendations:
Recommendations ITU-R F.1094, F.1108, F.1190, F.1495, S.523, S.671, S.735, S.1323,
S.1432, M.1313, M.1460, M.1461, M.1462, M.1463, M.1464, M.1465, M.1466, M.1638,
M.1644, M. 1652, M.1849, BS.412, BT.655, BT.1368, BO.1297, BO.1444, M.687, M.1073,
M.1388, SM.851, M.1183, M.1231, M.1232, M.1234, M.1478, SA.609, SA.1157, SA.1155,
SA.1396, SA.363, RS.1263, SA.514, SA.1026, SA.1160, SA.1163, RS.1029, RS.1166,
RA.769, BS.1660, BS.216, BS.560, BS.1786 and BT.1786.
In line with the scientific works and standardization activities toward implementation of CRs,
international treaties, such as ITU, have put already the matter under the consideration.
Resolution 956 (WRC-07) resolves to invite the ITU-R to study whether there is a need for
regulatory measures related to the application of software defined radio and cognitive radio
system technologies. Therefore, a new Agenda Item 1.19 proposed for the work of World
Radiocommunications Conference in 2012 (WRC-12) just to discuss the possibility of a
harmonized action. Fortunately, there is a report published by the ITU-R responsible study
groups which is addressing a good progress. Definitions for Software Defined Radio (SDR)
and Cognitive Radio Systems (CRs) have been developed and are published in Report ITU-R
SM.2152.
Cognitive radio is a revolutionary technology that aims for remarkable improvements in
efficiency of spectrum usage. It will change the way the radio spectrum is regulated, but also
requires new enabling techniques.
3. Different definitions of CRs
There are several definitions of CR and definitions are still being developed both in academia
and through standards bodies, such as FCC, IEEE-1900 and the SDR Forum. Summarizing
Mitola, a full CR can be defined as “…a radio that is aware of its surroundings and adapts
intelligently”. This may require adaptation and intelligence at all the 7 layers of the ISO
model. Full Cognitive Radios do not exist at the moment and are not likely to emerge until
2030, when fully flexible SDR technologies and the intelligence required to exploit them
cognitively can be practically implemented. We expect basic intelligent reconfigurable CR
prototypes to emerge within the next five years. Some devices available already have some
elements of CR. Examples include adaptive allocation of frequency channels in DECT
wireless telephones, adaptive power control in cellular networks and multiple input multiple
output (MIMO) techniques.
Under the framework of World Radiocommunication Conference 2012 Agenda item 1.19,
based on the results of ITU-R studies, in accordance with Resolution 956 (WRC-07)”, ITU-R
Working Party 1B has developed definition of Cognitive Radio System (CRS). The following
definition have been published in Report ITU-R SM.2152
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“Cognitive Radio System (CRs) is a radio system employing technology that allows the
system to obtain knowledge of its operational and geographical environment, established
policies and its internal state; to dynamically and autonomously adjust its operational
parameters and protocols according to its obtained knowledge in order to achieve predefined
objectives; and to learn from the results obtained.”
Despite of existence of diverse definitions by different persons and groups, actually there is
no other definition that adds to the concepts given in above definitions.
4. Overview of Cognitive Radio
This section will describe the Functions and components of cognitive radio and Potential
applications of cognitive radio. In addition Key benefits and challenges of CR will be
discussed.
4.1 Functions and components of Cognitive Radio
The main goal of cognitive radio is to provide adaptability to wireless transmission through
dynamic spectrum access so that the performance of wireless transmission can be optimized,
as well as enhancing the utilization of the frequency spectrum. The major functionalities of a
cognitive radio system include spectrum sensing, spectrum management, and spectrum
mobility. Through spectrum sensing, the information of the target radio spectrum (e.g. the
type and current activity of the licensed user) has to be obtained so that it can be utilized by
the cognitive radio user. The spectrum sensing information is exploited by the spectrum
management function to analyze the spectrum opportunities and make decisions on spectrum
access. If the status of the target spectrum changes, the spectrum mobility function will
control the change of operational frequency bands for the cognitive radio users. Based on the
described functions, Figure 1 depicts the components of a typical cognitive radio.
Figure 1 Components in a cognitive radio node[9].
4.2 Potential applications of Cognitive Radio
Cognitive radio concepts can be applied to a variety of wireless communications scenarios, a
few of which are described below:
• Next generation wireless networks: Cognitive radio is expected to be a key technology for
next generation heterogeneous wireless networks. Cognitive radio will provide intelligence to
both the user-side and provider-side equipments to manage the air interface and network
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efficiently. At the user-side, a mobile device with multiple air interfaces (e.g. Wi-Fi, WiMAX,
cellular) can observe the status of the wireless access networks (e.g. transmission quality,
throughput, delay, and congestion) and make a decision on selecting the access network to
connect with. At the provider-side, radio resource from multiple networks can be optimized
for the given set of mobile users and their QoS requirements. Based on the mobility and
traffic pattern of the users, efficient load balancing mechanisms can be implemented at the
service provider’s infrastructure to distribute the traffic load among multiple available
networks to reduce network congestion.
• Coexistence of different wireless technologies: New wireless technologies (e.g. IEEE
802.22-based WRANs) are being developed to reuse the radio spectrum allocated to other
wireless services (e.g. TV service). Cognitive radio is a solution to provide coexistence
between these different technologies and wireless services. For example, IEEE 802.22-based
WRAN users can opportunistically use the TV band when there is no TV user nearby or when
a TV station is not broadcasting. Spectrum sensing and spectrum management will be crucial
components for IEEE 802.22 standard-based WRAN technology to avoid interference to TV
users and to maximize throughput for the WRAN users.
• EHealth services: Various types of wireless technologies are adopted in healthcare services
to improve efficiency of the patient care and healthcare management. However, using
wireless communication devices in healthcare application is constrained by EMI
(electromagnetic interference) and EMC (electromagnetic compatibility) requirements. Since
the medical equipments and bio signal sensors are sensitive to EMI, the transmit power of the
wireless devices has to be carefully controlled. Also, different biomedical devices (e.g.
surgical equipment, diagnostic and monitoring devices) use RF transmission. The spectrum
usage of these devices has to be carefully chosen to avoid interference with each other. In this
case, cognitive radio concepts can be applied. For example, many wireless medical sensors
are designed to operate in the ISM (industrial, scientific, and medical) band, which can use
cognitive radio concepts to choose suitable transmission bands to avoid interference.
• Intelligent transportation system: Intelligent transportation systems (ITS) will increasingly
use different wireless access technologies to enhance the efficiency and safety of
transportation by vehicles. Two different types of communications scenarios arise in an ITS
system – vehicle-to-roadside (V2R) communication and vehicle-to-vehicle (V2V)
communication. In vehicle-to-roadside communications, information is exchanged between
the roadside unit (RSU) and the onboard unit (OBU) in a vehicle. In vehicle to-vehicle
communications, a special form of ad hoc network, namely, a vehicular ad hoc network
(VANET), is formed among vehicles to exchange safety-related information. High mobility of
the vehicles and rapid variations in network topologies pose significant challenges to efficient
V2R and V2V communications. Cognitive radio concepts can be used in both OBUs and
RSUs so that they can adapt their transmissions to cope with the rapid variations in the
ambient radio frequency environment. With multi-radio capabilities at the OBUs, they should
be able to adaptively choose the radio to communicate with the RSUs.
• Emergency networks: Public safety and emergency networks can take advantage of the
cognitive radio concepts to provide reliable and flexible wireless communication. For
example, in a disaster scenario, the standard communication infrastructure may not be
available, and therefore, an adaptive wireless communication system (i.e. an emergency
network) may need to be established to support disaster recovery. Such a network may use the
cognitive radio concept to enable wireless transmission and reception over a broad range of
the radio spectrum.
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4.3 Key benefits of CR
The main specific benefit of full CR is that it would allow systems to use their spectrum
sensing capabilities to optimize their access to and use of the spectrum. From a regulator’s
perspective, dynamic spectrum access techniques using CR could minimize the burden of
spectrum management whilst maximizing spectrum efficiency. Additional benefits from the
development of SDR, coupled with basic intelligence, are optimal diversification enabling
better quality of service for users and reduced cost for radio manufacturers.
4.4 Spectrum Regulation Changes
Cognitive radio means not only improving technology, it also requires fundamental changes
in the way radio spectrum is regulated. Depending on the regulatory status of the radio
systems that operate in the same spectrum, cognitive radios share spectrum with radio systems
that are designed to access spectrum with different priorities. To reflect this priority, licensed
and unlicensed radio systems are sometimes referred to respectively as primary and secondary
radio systems. Either licensed radio systems designed to operate in exclusively assigned
bands, or unlicensed radio systems designed to live with some interference from dissimilar
radio systems may share spectrum with cognitive radios. Sharing with primary radio systems
is referred to as vertical sharing, and sharing with secondary radio systems is referred to as
horizontal sharing. Apparently, dissimilar cognitive radios that are not designed to
communicate with each other may also share the same spectrum. This is another common
example of horizontal sharing, because the dissimilar cognitive radio systems have the same
regulatory status, i.e. similar rights to access the spectrum. For vertical and horizontal
sharing, a cognitive radio must be capable of detecting under-utilized spectrum, i.e. spectrum
opportunities, also referred to as “white space” spectrum.
Typically, spectrum opportunities change over time and vary depending on the location of the
cognitive radio. To protect the licensed radio systems and their services in vertical sharing
scenarios, other radio systems may assist cognitive radios in identifying spectrum
opportunities. Hence, regulation would be changed towards dynamic spectrum assignment.
Even more flexibility and a higher level of freedom could be envisioned for horizontal
sharing, eventually with less predictable outcome. Here, the cognitive radios would identify
opportunities autonomously. To avoid chaotic and unpredictable spectrum usage as in today’s
unlicensed bands, advanced approaches such as “spectrum etiquette” and “value-orientation”
are helpful. Spectrum etiquette is today discussed for existing unlicensed bands in various
regulatory bodies and standardization groups.
To guarantee fairness and efficiency, the way a cognitive radio makes decisions must be
traceable for regulators. In traditional radio systems, algorithms for spectrum management,
such as power control and channel selection, are implemented in many radio devices, but are
vendor-specific and not visible to the outside world, for example regulators. As a result,
today’s standards and regulation have to drastically constrain parameters like power levels
and frequency ranges for operation, to achieve a minimum level of interoperability, spectrum
efficiency, and fairness in spectrum access. The unique characteristic of cognitive radios on
the other hand is that their radio resource management algorithms are weakly constrained by
standards or regulation. This implies that the entire algorithms for decision-making in
spectrum management have to be visible to the outside world, and control mechanisms for
regulators have to be developed
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5. Regulatory and Standards aspects
5.1 Worldwide Regulations
FCC, OFCOM, CEPT, and other government agencies have made various rules to promote
both economic growth and more efficient use of spectral resource. In this section, we will give
a review on the regulatory activities on cognitive radio around the world
5.1.1 FCC
USA FCC started the rule making with a spectrum policy task force report in Nov. 2002, later
released its Notice of Proposed Rule Making in Dec. 2003 and May 2004 and issued the First
Report and Order in Oct. 2006. After 18 months of FCC OET testing on prototypes submitted
by Adaptrum, Institute for Infocomm Research (I2R), Microsoft, Motorola, Philips, FCC
released its Second Report and Order in Nov. 2008. In the following, the essential parts of the
rules are described.
Unlicensed broadband devices that could be operated in the TV bands are classified by FCC
into two general functional categories: “personal/portable” devices and “fixed/access”
devices. The first category would generally be lower power unlicensed devices, such as Wi-Fi
like cards in laptop computers or wireless in-home local area networks. The second category
would be higher power unlicensed devices that are generally operated from a fixed location
and may be used to provide commercial services such as wireless broadband internet
accesses. FCC would allow both types of operations in the TV spectrum, provided that
appropriate measures are taken to ensure that operations are limited to unused TV channels.
FCC also proposed different interference avoidance requirements for these two different types
of unlicensed devices. These proposals should provide flexibility to permit a wide range of
unlicensed broadband uses and applications, and at the same time ensure that the most
appropriate and effective mechanisms are in place to limit such unlicensed usage to only
unused TV channels.
FCC has proposed Protection contour and separation requirements, Sensing requirements,
Cognitive device RF requirements and Geo-location and database access requirements To
prevent interference to the primary users.
Possible TV band spectrum for cognitive radio usage in USA: All TVBDs are permitted to
operate in TV channels 21 to 36 (512-608 MHz) and channels 38-51 (614-698 MHz), except
some restricted areas where private land mobile services and commercial land mobile services
are used. TV channel 37 (608-614 MHz) is not allowed to protect radio astronomy and
wireless medical telemetry services on the channel. Operation in channel 2 (54-60 MHz),
channels 5 to 20 (76-88 MHz, 174- 216 MHz, 470-512 MHz) is permitted only for fixed
devices that communicate with each other.
5.1.2 OFCOM
In UK, 368MHz in UHF band (470-862MHz) is used for analog TV and 256MHz is reserved
for Digital Terrestrial TV (DTT) after the Digital Switch Over (DSO). The DSO, region by
region, started on 2007 and will be finished on 2012. Due to the DSO and re-allocations, from
2007 to 2012 there will be additionally 128 MHz frequency (112 MHz from DSO and 16
MHz from aeronautical radar and radio astronomy) available at different regions. There are
also possible white spaces in the DTT network, for example, the “interleaved spectrum” (e.g.
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Local TV, Programmer Making and Special Events (PMSE), Cognitive). In Feb 2009,
OFCOM released a proposal which allowed unlicensed cognitive access to the spectrum.
Based on the proposal, a wide range of applications such as high speed always-on broadband
could be operated by using the TV white spaces. To use the spectrum, any cognitive devices
must guarantee that the licensed users (including DTT and PMSE) are protected from harmful
interference.
OFCOM suggested three approaches for cognitive access: sensing, geo-location and beacon.
OFCOM recognizes that the three approaches have different advantages and disadvantages.
Sensing has the capability to make most effective use of the white space but the hidden
terminal problem may result in some residual probability of interference. Geo-location
requires a database, a self-locating capability for devices, and a frequently updating database
by licence holders to effective use of the white space. Beacons require an infrastructure to
transmit and needs a database to store the information to be transmitted. Currently OFCOM
thinks that the beacon approach is less effective compared with the other two approaches and
therefore will not consider it at this moment. OFCOM is currently working on the geolocation consultation. It may allow cognitive devices with geo-location capabilities to use the
TV white space.
5.1.3 CEPT
On June 2008, CEPT released a report (CEPT Report 24) titled: “A preliminary assessment of
the feasibility of fitting new/future applications/services into non-harmonized spectrum of the
digital dividend (namely the so-called “white spaces” between allotments)”. CEPT identifies
white space as a part of the spectrum, which is available for a radio communication
application (service, system) at a given time in a given geographical area on a non-interfering
non-protected basis with regard to primary services and other services with a higher priority
on a national basis. CEPT defines white spaces in the UHF band as any 8-MHz segments of
spectrum between active stations in a given area and in a given time. PMSE will continue to
have controlled access to white space spectrum to maintain its existing services in the UHF
band.
CEPT does not have conclusions on the feasibility of cognitive sharing schemes of cognitive
radio technology for white space devices. Any new white space applications will be used on a
non-protected non-interfering basis. Further studies are recommended to look into the
framework needed for the use of CR devices within white space spectrum. Currently the
CEPT SE43 group is defining the technical and operational requirements for the operation of
cognitive radio systems in the white spaces of the UHF broadcasting band (470-790 MHz)
such that incumbent radio services/systems are sufficiently protected.
5.2
Worldwide Standardization Activities
The major organizations which have made or are making various standards for cognitive radio
include IEEE, ITU, SDR forum, ETSI and ECMA. In this section, we summarize the major
standardization activities worldwide.
5.2.1 IEEE activities
5.2.1.1 IEEE 802.22 standard
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With relatively low levels of industrial noise and ionosphere reflections, reasonable antenna
sizes, and good non-line-of-sight (NLOS) propagation characteristics, the TV broadcast bands
in the high-VHF/low-UHF range are ideal for covering large areas in sparsely populated rural
environments. Starting with a Notice of Inquiry by the U.S. FCC in December 2002 exploring
the possibility of allowing access to the TV broadcast bands for license-exempt devices on a
noninterfering basis, and subsequently, a golden opportunity was created to develop a system
capable of using these frequency bands on a noninterfering basis to bring broadband access to
rural areas.
The development of the IEEE 802.22 WRAN standard is aimed at using cognitive radio
techniques to allow sharing of geographically unused spectrum allocated to the television
broadcast service. IEEE 802.22 WRANs are designed to operate in the TV broadcast bands
while ensuring that no harmful interference is caused to the incumbent operation (i.e., digital
TV and analog TV broadcasting) and low-power licensed devices such as wireless
microphones. IEEE 802.22 uses geo-location/database and spectrum sensing to understand the
spectral environment. In the first method, knowledge of the location of the cognitive radio
devices combined with a database of licensed transmitters is used to determine which
channels are locally available. In spectrum sensing, the received signals are analyzed to
identify which channels are occupied by licensed transmission.
5.2.1.2 IEEE SCC41 activities
In coordinating the work of Cognitive Radio, IEEE has created IEEE Standards Coordinating
Committee 41 (SCC41) to address the issues related to the deployment of next generation
radio systems and advanced spectrum management.
IEEE SCC41 believes that CR standardization can be considered an interactive process of
business case, technology, and policy. Although research is typically thought of as a
technological component only, business cases and policy research must be taken into account,
especially in CR standardization.
The IEEE SCC41 was later preceded by IEEE 1900 task force which started in the first
quarter of 2005. This task force consists of IEEE Communications Society and IEEE
Electromagnetic Compatibility Society. In April 2007, the IEEE Standards Board approved
the reorganization of the IEEE 1900 effort as Standards Coordinating Committee 41 (SCC41),
Dynamic Spectrum Access Networks (DySPAN). The IEEE SCC41 is divided into four
working groups (WGs) initially and added two more working groups to it. Each of the WG is
responsible to formulate standards for different aspects of cognitive radio denoted as IEEE
1900.x. The six WGs and their responsibilities are listed in Table 1.
Table 1: IEEE 1900 Working Groups.
Working Group
Responsibility
IEEE 1900.1
Standard definitions and concepts for spectrum management and advanced
radio system technologies.
IEEE 1900.2
Recommended practice for interference and coexistence analysis.
IEEE 1900.3
Recommended practice for conformance evaluation of software defined
radio software modules
IEEE 1900.4
Coexistence support for reconfigurable, heterogeneous air interfaces.
IEEE 1900.5
Policy language and policy architectures for managing cognitive radio for
dynamic spectrum access.
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IEEE 1900.6
Spectrum sensing interfaces and data structures for dynamic spectrum
access and other advanced radio communication systems.
5.2.1.3 Other IEEE standards
Following the FCC decision on opening up the TV bands for dynamic spectrum access, some
other IEEE groups like 802.11 and 802.16 started to make amendments to their established
standards for supporting the operations in TV White Spaces.
1) IEEE 802.11af –formed in January 2010 to adapt 802.11 to TV band operation
2) IEEE 802.16h: IEEE 802.16h develops coexistence mechanisms for license-exempt
operations of WiMAX at 2.4GHz and 5GHz, and supports the coexistence of such systems
with primary users. Through reducing the potential interference caused by such systems
sharing the same frequency bands, it targets improved user service experience and increased
robustness and efficiency of the spectrum use. A mechanism called Cognitive Radio Signaling
is introduced to help co-channel 802.16 base stations to mitigate interference.
3) IEEE 802.19: Many of the IEEE 802 wireless networks use unlicensed spectrum and hence
the issue of coexistence of different wireless networks within the same location needs to be
addressed. IEEE 802.19 is the Wireless Coexistence Technical Advisory Group (TAG) within
the IEEE 802 LAN/MAN Standards Committee dealing with such coexistence issue.
5.2.2 ETSI standardization initiative
ETSI Board created a Technical Committee for Reconfigurable Radio Systems (RRS) in Jan.
2008. The main purpose is to study the feasibility of standardization activities related to
Reconfigurable Radio Systems including SDR and CR. In Oct. 2009, ETSI’s RRS Technical
Committee released a series of ETSI Technical Reports (TR 102 838) that examine the
standardization needs and opportunities. The reports summarize the feasibility studies carried
out by the committee and present the recommended topics for standardization. Following the
completion of the feasibility studies, standardization of RRS is now getting underway.
5.2.3 SDR Forum standardization initiative
In September 2008, SDR Forum published a report on “Cognitive Radio Definitions and
Nomenclature” (SDRF-06- P-0009-V1.0.0). This report identifies components and collects
working definitions for many of the technologies and techniques related to cognitive radios.
The Cognitive Radio Work Group (CRWG) under SDR Forum is also working on a report
entitled “Quantifying the Benefits of Cognitive Radio”, which is developed for understanding
the benefits of using cognitive radio technologies in next generation wireless systems.
Since September 2009, the SDR Forum has also started a test and measurement project to
develop a set of use cases, test requirements, guidelines and methodologies for SDR/CR
unique functions required for secondary access to TV band spectrum. The project addresses
critical TVWS functions, e.g., spectrum sensing, interference avoidance, database
performance, and adherence to policy.
5.2.4 ECMA standard
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ECMA International is an industry association founded in 1961 and dedicated to the
standardization of Information and Communication Technology (ICT) and Consumer
Electronics (CE). In Dec. 2009, ECMA published a Standard called ECMA-392 entitled
“MAC and PHY for Operation in TV White Space” [34]. This Standard specifies a MAC sub
layer and a PHY layer for personal/portable cognitive wireless networks operating in TV
bands. This standard also specifies a MUX (a session management protocol) sub-layer for
higher layer protocols. It also specifies a number of incumbent protection mechanisms which
may be used to meet regulatory requirements.
5.2.4
ITU Activities
International Telecommunication Union Radiocommunication sector (ITU-R) has started
activities on CRs, in particular on the measurement of spectrum occupancy and CR systems in
the mobile service, indicating that the inclusion of the CR concept into the future regulatory
framework is topical.
in Agenda Item 1.19, ITU-R decided “to consider regulatory measures and their relevance, in
order to enable the introduction of software-defined radio and cognitive radio systems, based
on the results of ITU-R studies, in accordance with Resolution 956. To address WRC-12
Agenda Item 1.19, Study Group 1 (Spectrum Management) has been assigned to be the lead
organizational entity within ITU-R. ITU-R Study Group 5 (Mobile, Radio Determination,
Amateur, and Related Satellite Services) Working Party 5A (Land Mobile Service Excluding
IMT-2000; Amateur and Amateur Satellite Services) will continue its work on the
development of an ITU-R Report entitled “Cognitive Radio Systems in the Land Mobile
Service.” This work is in response to two Questions on SDR and CRS assigned to ITU-R
Working Party 5A: ITU-R 230-2/5 (Software Defined Radios) and ITU-R 241-1/5 (Cognitive
Radio Systems in the Mobile Service)
In particular, the ITU Radiocommunication Assembly in 2007 sent out two questions related
to CR, namely Question ITU-R 233/1 and Question ITU-R 241/8. The Question ITU-R 233/1
on measurement of spectrum occupancy is studied in ITU-R SG 1 and the responses to this
question will be collected in ITU-R Recommendations. Question ITU-R 233/1 lists the
following issues to be studied:
1. What techniques could be used to perform frequency channel occupancy measurements,
including processing and presentation methods?
2. What techniques could be used to perform frequency band occupancy measurements,
including processing and presentation methods?
3. How can “occupancy” be defined for both, frequency channel as well as for frequency band
measurements, also taking into account, the size of the used filter and the values measured in
adjacent channels?
4. How can threshold levels be defined and applied in practical situations including dynamic
threshold levels?
The Question ITU-R 241/8 on CR systems in the mobile service is studied in ITU-R SG 5 and
the responses to this question will be collected into ITU-R Recommendations and/or Reports
which will be completed by the year 2010.
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The Question ITU-R 241/8 lists the following issues to be studied:
1. What is the ITU definition of cognitive radio systems?
2. What are the closely related radio technologies (e.g. smart radio, reconfigurable radio,
policy-defined adaptive radio and their associated control mechanisms) and their
functionalities that may be a part of cognitive radio systems?
3. What key technical characteristics, requirements, performance and benefits are associated
with the implementation of cognitive radio systems?
4. What are the potential applications of cognitive radio systems and their impact on spectrum
management?
5. What are the operational implications (including privacy and authentication) of cognitive
radio systems?
6. What are the cognitive capabilities that could facilitate coexistence with existing systems in
the mobile service and in other radiocommunication services, such as broadcast, mobile
satellite or fixed?
7. What spectrum-sharing techniques can be used to implement cognitive radio systems to
ensure coexistence with other users?
8. How can cognitive radio systems promote the efficient use of radio resources?
The next WRC in 2012 (WRC-12) has an agenda item (1.19) on SDR and CR:
“to consider regulatory measures and their relevance, in order to enable the introduction of
software-defined radio and cognitive radio systems, based on the results of ITU-R studies,
in accordance with Resolution 956 (WRC-07”; The methods to satisfy the agenda item related
to Cognitive Radio Systems (CRS) are as follows:
Under Method B1( No change to the Radio Regulations), technical and operational
considerations related to the CRS technologies implemented in any systems of a
radiocommunication service could be developed in ITU-R Recommendations and Reports as
appropriate.
Under Method B2 ( No change to the articles of the Radio Regulations and a Resolution
providing guidance for further studies on CRS) a WRC Resolution is developed to provide a
framework for guidance of the studies on technical and operational considerations related to
the CRS technologies implemented in any systems of a radiocommunication service leading
to ITU-R Recommendations and Reports as appropriate.
Under Method B3( No change to the articles of the Radio Regulations and a Resolution
dealing with the use of CRS and further studies within the ITU-R) a WRC Resolution is
developed to provide provisions for the implementation of CRS as well as a framework for
guidance of the studies on technical and operational considerations related to the CRS
technologies.
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6. Cognitive Radio implementation
6.1 Implementation issues
6.1.1 RF design
A primary technological concern in cognitive radio architectures, whether it be for wideband
sensing procedures or wideband multi-band communication mechanisms is the ability to
design linear and spectrally-agile components and architectures in the radio-frequency frontend of the transceiver. In a conventional radio design, some assumptions are made on the
interferers and, based on worst-case scenarios; the performance of the RF front-end is
specified with respect to selectivity and linearity. Conventional radios typically utilize a preselect filter at the receiver input to limit the interferers, which the active part must be able to
withstand. However, for a cognitive radio this approach is not very practical due to its
inherent need to flexibly select the radio frequency. Removing or relaxing the preselect filter
selectivity significantly exacerbates the problems due to interferers. All RF front-end
specifications cannot be directly mapped to circuit blocks without information on the
interferer scenarios. Some of the error generation mechanisms are complex and, in general, it
is a fairly involved task to find out the building block requirements that lead to adequate
receiver performance under all expected conditions. The fact that, in a cognitive radio, neither
the RF frequency nor the bandwidth is known in advance complicates the situation
considerably. Following the well-proven methods of receiver design and frequency planning
will lead to excessive circuit block requirements particularly in absence of a pre-select filter at
the receiver input. In order to deal with the more stringent performance requirements, a
cognitive radio should be designed to take advantage of its inherent capabilities. It should use
the information it possesses on the interferer situation and its own non-idealities to select the
RF frequency, not only based on spectrum occupancy, but also on the suitability of a given
frequency for communication. This will help in relaxing the circuit block requirements, so
that they do not become excessive, while not forcing the initial radio design to limit the
capabilities of the cognitive radio.
6.1.2 System On Chip Implementation
Designing the digital baseband processing of such an extremely agile system is a very
challenging task. The required processing power is huge in most of the functional unit and the
memory needs and memory bandwidths are also usually very high. But the two most difficult
aspects are probably: the partitioning of the system in hardware and software processing units,
and the system integration and the design of the embedded software.
The partitioning implies a deep study of the basic algorithms involved. The different
variations of a given function must be identified. As in most cases there are many different
implementation options, the design space to be explored is a large one. The output of this
algorithmic analysis is a set of highly flexible functional entities. The design of these entities
is less challenging. However, it strongly depends on the selected target technology.
The system integration phase is also a critical issue. Scheduling of the hundreds of different
tasks running on very different operating units requires an accurate modeling of their
dependencies, of their parallelization possibilities and of their timing-related constraints. The
entire platform is controlled by a complex embedded software application running on a set of
embedded CPU cores. The challenges here are those of a real-time constrained application in
the context of a multi-processor System on Chip architecture.
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6.2. Implementation examples
Research and development results on a software defined cognitive radio equipment that
consists of a hardware platform and a software platform have been introduced in [16]. The
hardware platform consists of a multi-band antenna supported from UHF band and 2-5 GHz
band, multiband RF unit, signal processing unit consists of FPGA and CPU boards. The
software platform consists of several managers that manage spectrum sensing and
reconfiguration of communication systems. The developed cognitive radio prototype
combined by hardware and software platforms senses the signal level (RSSI) over 400MHz6GHz bands and moreover identifies the system by using software packages and checks RSSI,
BER, connectivity, and so on. The software packages can configure specified wireless
communication systems and consist of physical layer, MAC/DLC layer, IP layer, and
application layer part of the systems.
Paper [23] presents a test bed for experimenting with Cognitive Radios at the physical and
link layer. The motivation for a test bed is provided by the need to validate various sensing
algorithms to prove non-interference to licensed users and to evaluate their performance with
well defined metrics. This test bed allows us to emulate Primary as well as Secondary Users
and enables the evaluation of the performance of various spectrum sensing schemes. The
2.4GHz spectrum was chosen for initial experimentation due to the availability of off-theshelf transmission equipment and the ability to emulate Primary Users in a controlled manner.
These 2.4GHz radios are connected to the Berkeley Emulation Engine 2 (BEE2), which is a
multi FPGA emulation platform. The FPGAs enable the implementation of complex signal
processing functions and the inherent parallelism of the FPGAs supports concurrent operation
of multiple radios. The Cognitive Radios can exchange sensing and setup information in a
timely manner since the BEE2 FPGAs are connected via high bandwidth low latency links.
Cognitive Radio (CR) equipments are radio devices that support the smart facilities offered by
future cognitive networks. So it is necessary to add inside the radio equipments some
management facilities for that purpose, and the paper[24] proposed architecture is called
HDCRAM (Hierarchical and Distributed Cognitive Architecture Management). It consists in
the combination of one Cognitive Radio Management Unit (CRMU) with each
Reconfiguration Management Unit (ReMU) distributed within the equipment. Each of these
CRMU is in charge of the capture, the interpretation and the decision making according to its
own goals.
More implementation examples can be found in [25,26,27]
7. Regulatory issues and Recommendations
Cognitive radio is a revolutionary technology that aims for remarkable improvements in
efficiency of spectrum usage. It will change the way the radio spectrum is regulated,
Basically, the main role of regulators is to ensure that cognitive radio devices don’t interfere
with the existing licensed services and if it happens, how to deal with it. Although cognitive
radio technology is said to be able to self manage spectrum usage, regulators around the world
are looking at it cautiously. There are still many issues that need to be resolved before the
technology is actually implemented for commercial use.
7.1
REGULATION
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Regulatory bodies must ensure that devices in the CR field conform to contemporary and
future requirements for radio equipment. To achieve this goal, a conformity assessment
apparatus must be developed using many components, such as equipment certification, quality
control, and field monitoring. Providing regulators with the standards they require to fulfill
their mandate is an area for many future projects. Regulation documents would describe
methods to measure the interference caused by CR and CNs, and quantify the intelligence of
such devices.
It is clear that there is much commonality in the new approaches to spectrum management and
regulation being discussed by the regulators in a number of countries around the world. The
commonalities include recognition of a need for a new approach to spectrum management, the
use of market mechanisms to accomplish spectrum management, recognition that new
technological innovations such as SDR, UWB. Policy-based adaptive radio and CR will be a
key part of the spectrum management paradigm shift, that the paradigm shift is a long-term
process (10 – 20 years), Any controversy associated with the spectrum management paradigm
shift is not likely to be primarily between regulators from different administrations the
controversy is more likely to be between the regulator and spectrum license holder for
specific portions of the RF spectrum.
This is particularly true for license holders who have paid large sums of money for their
licenses such as the license holders in the commercial wireless bands. Although the operator
had claimed “exclusive rights,” the FCC ruled that incumbent spectrum license holders do not
have the right to exclude new users from transmitting in their assigned bands.
7.2
Security
Although existing wireless security standards can be used in CR networks for certain aspects
(e.g., encryption), there are several unique challenges that arise merely due to the
opportunistic nature of spectrum access. For example, in order to accurately sense white
spaces, as well as to securely transmit this decision to all nodes in the secondary network, it is
not only necessary to design standalone optimal sensing techniques, but also authenticated
encryption enabled protocols that will allow a reliable, joint, and speedy decision for the
entire network. Hence, a more holistic approach is needed while designing the several
components of the CR network. A good design will result in accurate and secure primary user
(PU) detection, resilience to non-jamming denial of service (DoS) attacks on the secondary
user (SU), efficient and fair spectrum sharing, accurate authorization, and computational
efficiency. In order to understand the components needed to design a secure CR network, it is
necessary to understand the threats a CR network could face. The problem of spectrum
opportunity detection is intimately connected with the problem of detecting PU activity in any
given band. If this key functionality is not accurately implemented, one of the following
undesirable situations could occur.
7.3
Enforcement and certification - Certification of cognitive radio devices is a
challenge. First, it inherits the challenges of software certification because a cognitive radio is
likely to have a software component. Second, certification faces the challenging issue of
whether to certify a device or to certify a component. For example, a cognitive radio may
consist of multiple components, such as a policy reasoner, a sensing component, and frontend.
A possible example of certifying component is to certify a policy reasoner which is decoupled
from the radio platform. Such modular approaches can simplify the process.
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Third, certification faces the challenging issue of addressing the networking aspect of
cognitive networks. The network aspect can have both positive and negative impact on the
PU/SU interaction. For instance, it has been well established that cooperative sensing can
significantly improve the sensing performance. How can this positive effect be taken into
account in certification? On the other hand, a PU may require certain protection, say
interference below threshold. While a single SU device may not emit above-the-threshold
interference, a collective set of SUs may cause outage at the PU. How can this negative effect
be avoided in the certification process?
Enforcement is a related challenge. The current approach, with FCC being the main enforcer
with labor-intensive measurements, is not likely to scale to billions of cognitive devices with
much more flexibility, and therefore, much bigger potential for malfunction as well as
malicious usage. In fact, distinguishing between correct and faulty behavior can be a very
difficult program. Alternative solutions need to be considered, e.g., enlisting cognitive radios
to identify/report potential policy violations.
7.4 Protecting PU
Primary user (PU) protection is vital to the success of wide adoption of dynamic spectrum
access since no PU would accommodate SU access to its own detriment. This is also the
major concern of legacy spectrum holders. Most existing research has been focusing on
Listen-before-Talk (LBT) where secondary users sense the spectrum (potentially collectively)
before transmitting. Good progress has been made both in theoretical domain and prototype
testing. However, LBT has its limitations. Because it focuses on the transmitter rather than the
receiver, LBT needs to be conservative to protect PU against SU interference. For instance,
the threshold for the LBT devices was set at 30dB below the DTV reception threshold in the
FCC TV white space testing.
In order to overcome these limitations, the research community should consider other options.
For example, in the context of TV white space, FCC database has been used (sometimes in
conjunction with sensing) to predict spectrum availability because TV broadcast locations are
fixed and schedule predetermined. Another option is to focus on receivers, more specifically
the observability of receivers. There are both active and passive approaches. An example of
the active approach is to introduce a (beacon) device on the receiver to announce its presence.
This may be easier and less expensive than trying to sense for transmitters and it avoids
hidden terminal problems. It has for example been demonstrated that it is possible for a low
cost device to detect when a TV set is on, and then announces itself. This type of approach
has the potential to enable TV white space reuse in metropolitan areas, where the spectrum
demand is high and unused TV band is scarce. Research is needed to study/quantify the
tradeoff between the performance gain and the complexity to enable receivers, as well as
security implications.
7.5. Guidelines and Recommendations
 There is a need to identify a timeline for the time phased transition to the new
spectrum management paradigm. A roadmap should be developed for this transition
which considers legacy issues and special band issues.
 It is recommended that issues which need to be addressed by the regulatory bodies be
identified. It can be expected that the transition to a new spectrum management
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





approach will have differences in different administrations both in scope, and the
timeframe/roadmap for accomplishing the transition.
It is recommended that mechanisms to informally discuss at an international level the
spectrum management transition be put in place. This is in addition to the formal ITU
process.
Harmonizing the viewpoints, exchanging data and providing guidelines is partially an
important role for regulators and standardization bodies, and also for technical
community. For example there is need to harmonize terminology and reference
models. There is also possible danger of increasing fragmentation of the community
and terminology. The reference models are required for a suitable discussion. National
regulators could and most probably should play an important role on driving some of
the work towards harmonization.
There are regulatory dimensions that need to be considered, and many of those go
beyond simple spectrum regulation Including aspects of equipment, conformance
(responsibility), cognitive pilot channels, and interface regulation & standardization.
Further work is needed to analyze tradeoffs and potential risks and benefits that are
related to CR and SDR technologies. There are only vague understandings on the scale
of costs that may be coming from new technology deployment and increased
interference risks. Part of the SWOT analysis should be also to consider a number of
different approaches.
In the near term a regulatory framework should be developed that encourages research
and the development of CR. For example, allocating a block of spectrum for CR
control and enabling secondary licensing, would achieve this.
CRs will require software-based spectrum policies. These policies will become an
integral part of the radio device. Regulators will be required to define these policies,
which will then be coded in the CR policy box. It is essential, therefore, for a regulator
to keep abreast of software policy development and certification issues.
8. Summary and Conclusions
Cognitive radio offers great benefits to all members of the radio community from regulators
to users. In terms of spectrum regulation, the key benefit of CR is more efficient use of
spectrum, because CR will enable new systems to share spectrum with existing legacy
devices, with managed degrees of interference. There are significant regulatory, technological
and application challenges that need to be addressed. Main challenges in summery are: First,
ensuring that CRs do not interfere with other primary radio users i.e. solving the hidden node
problem. Second, because CR relies on SDR, all the security issues associated with SDR,
such as authenticity, air-interface cryptography and software certification etc, also apply. The
third challenge is control of CRs. It is not clear how, or if, these problems can be solved.
Some regulators have allocated test bands for CR, to encourage development of CR
technologies in their national markets and elsewhere. One of the most important issues is band
sharing. There are two potential routes to band sharing. Either, the legacy spectrum holder
(i.e. the primary user and original licence holder) makes an agreement directly with a third
party organization (the secondary user or band sharer). The terms on which the spectrum
would be shared would be outlined and agreed between them and there would be no
regulatory involvement in either setting safety criteria, monitoring that safety criteria were
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being complied with, or imposing penalties if they were not kept. Alternatively, band sharing
in certain spectrum bands could be mandated by the regulator. In this case, it would be the
regulator’s responsibility to outline safety criteria, ensure that the primary user did not suffer
from interference as a result of the secondary user, monitor interference levels and impose
penalties if they were exceeded. In this case, the regulator would need to be convinced that
the benefits of Cognitive Radio in terms of spectral efficiency, would out-weigh the disbenefits – in terms of interference and market disruption.
Whether the further development of CR is enabled by the allocation of test bands, or through
the use of licence-exempt spectrum, or through band sharing of public or private spectrum
allocations, the regulator’s role will be to ensure that both legacy licensees and spectrum
sharers are able to operate effectively without compromising the rights and integrity of each
others’ systems.
The creation of the appropriate spectrum environment for CR will involve the development of
spectrum databases, of spectrum monitoring facilities and of software spectrum policies.
These will be required by the emerging market for reconfigurable radios, expected to develop
in the next 5 to 10 years, as standards mature.
The distinctive and intelligent features of cognitive radio do raise the question as to whether
cognitive radio can take over the spectrum management functions from communications
regulators. The answer is no. The role of the regulator is still needed and its role is necessary
to provide regulations, which would facilitate the use of cognitive radio. It cannot take over
the role of spectrum management in the near future also, while it efficiently uses spectrum, it
poses a challenge to regulators to mitigate interference caused by this technology.
In this document, the major functions and components of cognitive radio and implementation
issues are discussed and Regulatory Issues and Key Concepts are described in addition, the
state of art in regulatory and standardization activities on cognitive radio all over the world
are reviewed. It is seen that different countries may have different regulations. This seems to
be reasonable as different countries may have different white spaces, and faces different
social and economy challenges. However, this makes standardization in cognitive radio more
challenging. There are also different standards from different organizations. How these
standards can be harmonized is a big question in the near future. There must be some
consolidations in this area. The regulation and standardization are still ongoing and their final
impact remains unknown.
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9. REFERENCES
[1] FCC, “Second report and order and memorandum opinion and order,” in FCC 08-260, Nov. 2008.
[2] SDR Forum Website in http://www.sdrforum.org/.
[3] ETSI, “ETSI TR 102 838: Reconfigurable radio systems (RRS); Summary of feasibility studies and potential
stardization topics,” in http://www.etsi.org, Oct 2009.
[4] ECMA, “MAC and PHY for Operation in TV White Space,” in ECMA- 392, Dec. 2009.
[5] FCC, “First report and order and further notice of proposed rulemaking,” in ET Docket NOs. 04-186 and 02380, 21 FCC Rcd 12266, 12278, Oct. 2006.
[6] FCC, “Notice of proposed rulemaking,” in FCC 04-113, May 2004.
[7] http://www.ofcom.org.uk/consult/condocs/cognitive/.
[8] CEPT, “CEPT Report 24,” in http://www.erodocdb.dk.
[9] E. Hossain, D. Niyato and N. Han “Dynamic Spectrum Access and Management in Cognitive Radio
Networks”, Cambridge university press, 2009.
[10] J. Mitolla and G. Q. MaGuire, Jr., "Cognitive Radio: Making Software Radios More Personal," IEEE Pers.
Commun., Aug. 1999, pp. 13-18.
[11] S. Haykin, "Cognitive Radio: Brain-Empowered Wireless Communications," IEEE JSAC, Feb. 2005, pp.
201-20
[12] C. R. Stevenson, G. Chouinard, Z. Lei, W. Hu, S. J. Shellhammer, W. Caldwell “IEEE 802.22: the first
cognitive radio wireless regional area network standard, IEEE Communications Magazine”, Volume
47,Jan.2009, pp. 130-138
[13] Fabrizio Granelli,Przemyslaw Pawelczak, R. Venkatesha Prasad, Delft K. P. Subbalakshmi and
Rajarathnam Chandramouli, Stevens James A. Hoffmeyer, Western H. Stephen Berger, “Standardization
and Research in Cognitive and Dynamic Spectrum Access Networks: IEEE SCC41 Efforts and Other
Activities” IEEE Communications Magazine • January 2010, pp 71-79.
[14] Grande, L.M,”IEEE Dynamic Spectrum Access policy standards work”, Military Communications
Conference, 2009. MILCOM 2009. IEEE, 18-21 Oct. 2009pp.1-4
[15] M. Matinmikko, M. Mudstone, H. Sarvanko, M. Höyhtyä, A. Hekkala “A motivating overview of cognitive
radio: Foundations, regulatory issues and key concepts”, Cognitive Radio and Advanced Spectrum
Management, 2008. CogART 2008. First International Workshop on , 2008,pp. 1 – 5
[16] Hiroshi Harada, “Research and development on cognitive and software radio technologies, Devices and
hardware platform”, National Institute of Information and Communications Technology (NICT)
[17] Advanced Wireless Networks Cognitive, Cooperative and Opportunistic 4G Technology, 2009 John Wiley.
[18] Peha J. M., “Approaches to Spectrum Sharing,” IEEE Communications Magazine, Feb. 2005.
[19] IEEE 1900 in http://www.ieeep1900.org.
[20]C. Stevenson, G. Chouinard, Z. D. Lei, W. D. Hu, S. Shellhammer, and W. Caldwell, “IEEE 802.22: the first
cognitive radio wireless regional area network standard,” IEEE Communications Magazine, vol. 47, no. 1,
pp. 130–138, 2009.
[21] “Cognitive Radio Technology”A Study for Ofcom , QINETIQ/06/00420 Feb. 2007
[22] Cheng-xiang Wang; Hsiao-hwa Chen; Xuemin Hong; Guizani, M “Cognitive radio network
management”, Vehicular Technology Magazine, IEEE, Volume: 3, 2008,pp.28-35.
[23] S. M. Mishra , D. Cabric , C. Chang , D. Willkomm , B. V. Schewick , A. Wolisz , R.. W. Brodersen,
“A real time cognitive radio testbed for physical and link layer experiments”, IEEE Symposium of New
Frontiers in Dynamic Spectrum Access Networks, 2005
[24] Christophe Moy, “High-Level Design Approach for the Specification of Cognitive Radio Equipments
Management APIs”, Journal of Network and Systems Management, Volume 18, 2010, pp. 64 - 96
[25] V. Turunen, M. Kosunen, A. Huttunen, S. Kallioinen, P. Ikonen, A. Pärssinen and J. Ryynänen,
“Implementation of Cyclostationary Feature Detector for Cognitive Radios”, , 4th International
Conference on Cognitive Radio Oriented Wireless Networks and Communications (Crowncom), 2009.
[26] K. van Berkel, D. van Kampen, P. Kourzanov, O. Moreira, A. Piipponen, K. Raiskila, S. Slotte, M. van
Splunter and T. Zetterman, Multiradio, “Scheduling and Resource Sharing on a Software Defined Radio
Computing Platform”, SDR Forum 2009.
[27] T. Rapinoja, K. Stadius, L. Xu, S. Lindfors, R. Kaunisto, A. Pärssinen and J. Ryynänen, “A Digital
Frequency Synthesizer for Cognitive Radio Spectrum Sensing Applications”, IEEE Transactions on
Microwave Theory and Techniques, Volume 58, Issue 5, 2010.
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